Decorative and functional flame generating appliances often rely on combustion of natural gas, propane, and other liquid petroleum fuels to generate a flee. Known methods of flame supervision involve various automatic thermal sensing devices such as thermocouples and optical means of determining flame condition. Many fireplaces use a thermopile which, when heated, generates a small voltage which is applied to a relay to hold open a valve controlling the flow of gas to the fireplace. Such fireplaces typically use a pilot flame which is ignited by manually controlling the gas flow by holding open the valve and igniting the gas by means of a piezo electric spark generator. The thermopile is arranged in proximity to the pilot burner. Once the pilot flame has been established sufficiently long to heat up the thermopile, the thermopile voltage will act to hold open the valve controlling the supply of gas thereby enabling the pilot flame to remain lit when the operator releases manual control of the valve and enabling the operator to activate the main burner. When the main burner selection is made, gas flows through the burner bar which then ignites from the established pilot flame.
One problem with the above method is that when the pilot or main flame condition has been lost, it can take up to three minutes of cool down time before the thermopile will cease generating current so as to shut off the gas supply valve. This allows sufficient time to establish a high concentration of combustible gas in the combustion chamber, particularly if it is closed or sealed. Since most of the gases involved are colourless and in many cases undetectable, dangerous combustion conditions may fail to be recognized, resulting in hard lights or explosions occurring. To prevent this from occurring, it is also known to provide electronic flame supervision using an AC rectification system to monitor the presence of a flame.
A flame rectification system converts alternating current into direct current. The system applies an AC voltage to the flame rod and after the pilot flame is ignited the gas molecules between the flame rod and ground becomes ionized and have the ability to conduct an electrical current. Due to the difference between the grounding area and the flame rod size, the current through to flame flows mostly in one direction. This process results in a pulsating direct current which the flame monitoring circuit in the module is designed to accept. The system is adapted to respond only to this direct current in detecting the presence of flame.
This approach has the advantage of rapid shut off (unlike the thermopile), however, such systems may fail altogether during power outages when operation of the fireplace may be desired.
One object of this invention is to provide a rapid flame failure response which does not depend on an AC supply or a flame rectification system. Another object of this invention is to provide a rapid flame failure response in conjunction with a thermopile based flame monitor as a complete system and which may be retrofitted to a thermopile based fireplace.